1. Field of the Invention
This invention relates to printing ink compositions and, in particular, to printing ink compositions which are suitable for use in dry-planographic printing.
2. Description of the Prior Art
Recently, various methods have been proposed as a result of extensive investigational efforts directed toward the development of dry-planographic printing processes in which no dampening water is employed. For example, the most important method from the standpoint of practice was a method using planographic printing plates prepared by providing films of a non-sticky substance, such as silicone rubber, on the non-image areas, whereby printing with an oil-based printing ink in the absence of the water or dampening solutions is possible (see, for example, U.S. Pat. No. 3,511,178). One of the critical disadvantages in those dry-planographic printing plates has been the occurrence of scumming on the non-image areas when a conventional printing ink was employed.
In order to overcome this disadvantage, an improvement was proposed to enhance the tack of the printing ink by increasing its cohesion, which, however, results only in a very much increased piling of paper powder to a degree such that no practical printing has been possible.
A further improvement was proposed to formulate a printing ink composition suitable for dry-planographic printing, containing from 0.1 to 10% by weight of an organopolysiloxane (see, for example, Japanese Patent Publication No. 50-11287). This organopolysiloxane-containing ink composition, however, experimentally was found to be defective with respect to stability, bringing about the separation of the organopolysiloxane in the ink composition, since this composition was formulated by merely blending the organopolysiloxane with other materials.
The prior art printing ink compositions are not able to contain organopolysiloxane in high amounts, because there is a tendency for much of the organopolysiloxane content to cause increases in the viscosity of the ink composition, insufficient dispersion of pigments in the ink composition and other undesirable effects, as well as separation of the organopolysiloxane in the ink composition. Accordingly, the prior art ink compositions have had to have a low content of organopolysiloxane, and this eventually works to decrease the repellency of the ink composition, resulting in the occurrence of scumming on the printing plates.
The inventors of the present invention have conducted extensive studies on the improvement of the printing ink composition suitable for use in dry-planographic printing. They had held a basic concept that the problem should be a matter of interrelationship between the printing ink compositions and the printing plates. As a result, they have proposed printing ink compositions for dry-planographic printing containing a silicone-modified alkyd resin as disclosed in U.S. Pat. Nos. 3,945,957 and 3,948,827 or a silicone-modified vegetable oil as disclosed, for example, in Japanese Patent Disclosures No. 51-21907, having very good printability in the actual printing and capable of giving excellent printed copies.
These proposed printing ink compositions are, indeed, suitable for dry-planographic printing on a sheet-fed offset printing press. Such compositions exhibit a very good printability with excellent printed products, but the same compositions are defective when employed in a long-run printing on a high-speed web offset printing press, due to decreased ink tack and increased tendency to scumming. Scumming alone may be prevented from taking place during a long-run printing operation on the printing press by using an ink designed to have a higher initial ink tack, but such high tack tends to give rise to insufficient spread and unfavorable piling of the ink on the inking roller.